On 28/10/2013, at 11:41 PM, Robert Kjettrup <robert@stvmayday.dk> wrote:
>
> Thanks Chris for the tip about NOT using LZW compression on 16bit tiff files. Never thought about that could be a problem, i always leave this setting on. But dosent this more sound like a bug?

It's not a bug, it's just the way the algorithm was designed.  LZW is a very old and primitive form of compression and was specifically designed for 8 bit files.  It looks for patterns or values that repeat themselves a lot, and builds up a table of these repeated values.  In an 8 bit file where each value is restricted to 0 - 255 there's a high probability that sequences of numbers will repeat themselves, especially for things that have an underlying structure like language.  LZW can still do very well on text files, for such an old algorithm.  But in a 16 bit file the range of values is so much bigger (0-65535) that the probability of having specific sets of numbers that repeat falls dramatically, and the table becomes an overhead.

I imagine that for simple images, such as mattes, then LZW would still offer impressive file size advantages in 16 bit mode.  But for natural images such as photographs then it just doesn't work very well, and it's due to the 8 bit nature of the algorithm's origins.

I suppose it's a case of YMMV, but in general LZW isn't recommended for 16 bit tiffs.

-Chris
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